Apparatus for chemical or medical use and piping materials in semiconductor manufacturing plants, among others, are generally contacted to chemicals and the like on the occasion of use thereof and, therefore, are desired to be resistant to chemicals and the like and to corrosion. When the substrates constituting such apparatus, piping materials and so forth are protected by a corrosion-resistant lining, for instance, they can be provided with corrosion resistance.
Among the lining members each provided with a lining, there are ones in which fluororesins, in particular ethylene/tetrafluoroethylene copolymers [ETFEs], are utilized since they are easier to handle as compared with the conventional glass linings and have good resistance to chemicals such as acids, alkalis, oxidizing and reducing agents and various solvents.
When used for lining members, ETFEs are applied generally in the form of coatings such as powder coatings and, in this case, primers to be applied beforehand, as undercoats, to substrates are generally regarded as unnecessary, hence are not used. ETFEs are now in wide use in view of such simplicity in using them.
ETFE-based coatings comprising an ETFE are applied to articles to be coated and then baked for forming s coating film. Generally, the resulting films are desired to have a certain thickness in terms of being provided with corrosion resistance. In electrostatic coating processes, for instance, the steps of application and baking at a high temperature are repeated several times.
While ETFEs are by nature excellent in chemical resistance, among others, they are poor in adhesion to substrates and, therefore, when they are applied in the form of coatings to give relatively thick films, there arises a problem that the coatings may develop coating film defects such as cracks.
The formation of cracks results in a decrease in corrosion resistance. For avoiding cracking, it is impossible to increase the film thickness, hence the corrosion resistance has to remain poor. Thus, any ETFE-lining member satisfactory in corrosion resistance has not been obtained as yet.
Although ETFEs are excellent in heat resistance and show a certain degree of adhesiveness just after application thereof as coatings, their adhesiveness decreases under severe conditions, such as hot water or high temperature conditions, to cause problems, namely coating film defects, such as cracks in coating films and peeling of f from articles coated therewith.
ETFE-based coatings are sometimes applied to articles to be coated, which comprise chromium-containing metals such as stainless steels. Since chromium-containing metals promote the decomposition of ETFEs, there are problems on the application of the coating to articles to be coated.
To solve such problems and effectively use ETFE-based coatings, primer development has become desired. As a primer for ETFE-based coatings, there are known PPS-based primers comprising polyphenylene sulfide [PPS] and a polyamideimide resin.
However, no solvent is available for dissolving such PPS-based primers, hence these occur as solids. It is difficult to apply them onto articles to be coated evenly and uniformly and, as a result, voids between the primer and the substrate are readily occurred, resulting in peeling. Thus, they are incapable of completely solving the insufficient adhesion and coating film defect problems mentioned above.
ETFEs are generally high in alternating-copolymer character, the mole ratio between ethylene and tetrafluoroethylne is 1:1, and the fluorine content is 59.4% by mass. However, ETFEs higher in fluorine content for improving in gloss, transparency and flame retardancy have been desired for recent years. When the fluorine content is high, the adhesion to substrates is weak and, accordingly, coating film defects tend to appear. The demand for improvement in adhesion is thus increasing.